Single silicon wafer micromachined thermal conduction sensor

Abstract

A single silicon wafer micromachined thermal conduction sensor is described. The sensor consists of a heat transfer cavity with a flat bottom and an arbitrary plane shape, which is created in a silicon substrate. A heated resistor with a temperature dependence resistance is deposed on a thin film bridge, which is the top of the cavity. A heat sink is the flat bottom of the cavity and parallel to the bridge completely. The heat transfer from the heated resistor to the heat sink is modulated by the change of the thermal conductivity of the gas or gas mixture filled in the cavity. This change can be measured to determine the composition concentration of the gas mixture or the pressure of the air in a vacuum system.

What is claimed is: 
     
       1. A method of manufacturing a single silicon wafer micromechanical thermal conduction sensor comprising steps of:
 providing a silicon substrate having a resistivity ranging from 0.1 to 0.001 ohm-cm and a crystal orientation; 
 depositing a silicon nitride layer on the surface of the silicon substrate by LPCVD (low pressure chemical vapor deposition) which has a thickness in the range of 2000 to 3000 Angstroms; 
 performing first lithographic process including forming a photoresist pattern with a square or a circles shape window in the silicon nitride layer and etching exposed silicon nitride layer in the window; 
 performing anodization in a HF solution to convert the exposed single crystal silicon layer of the silicon substrate into a porous silicon layer; 
 depositing first insulating silicon nitride layer on the surface of the silicon substrate including the surface of the porous silicon layer by LPCVD; 
 depositing a polysilicon layer on the surface of the first insulating silicon nitride layer by LPCVD; 
 performing second photolithography process for creating a polysilicon pattern on the top central region of the porous silicon layer; 
 perform a third fabrication process by depositing second insulating silicon nitride layer coating the surface of the silicon substrate including the surface of the polysilicon pattern by LPCVD; 
 depositing a passivation layer over the surface of the silicon substrate by LPCVD or PECVD (plasma enhanced chemical vapor deposition); 
 performing fourth lithographic process to reveal the bonding pads; 
 performing fifth lithographic process to create at least one opening in the insulating layer so as to reveal its beneath porous silicon layer; 
 etching the porous silicon layer through the openings in a diluted HF solution so as to form a heat transfer cavity having a flat bottom parallel to the surface of substrate, a frame having a side curved wall surrounding the cavity, a thin film bridge suspending over the cavity and having a central section and at least two side sections on the two opposite sides of the central section, which extend to the edge of the frame; and 
 perform a sixth fabrication process creating a heated resistor on the top surface of the polysilicon pattern and a temperature sensor on the periphery top surface of the porous silicon layer by a lift-off process and metal deposition by E-beam evaporation or sputtering. 
 
     
     
       2. The method of manufacturing the single silicon wafer micromechanical thermal conduction sensor as cited in  claim 1 , wherein said porous silicon layer has a thickness ranging from 5 to 50 microns and is formed under the condition: HF solution consisting of one or two parts 48 wt HF and 1 part ethanol; anodic current density 40 to 80 mA/cm.sup.2. 
     
     
       3. The method of manufacturing the single silicon wafer micromechanical thermal conduction sensor as cited in  claim 1 , wherein said heat transfer cavity has a square plane shape with a side length ranging from 400 to 2000 microns and a vertical gap ranging from 5 to 50 microns. 
     
     
       4. The method of manufacturing the single silicon wafer micromechanical thermal conduction sensor as cited in  claim 1 , wherein said heat transfer cavity has a circle plane shape with a diameter ranging from 400 to 2000 microns and a vertical gap ranging from 5 to 50 microns. 
     
     
       5. The method of manufacturing the single silicon wafer micromechanical thermal conduction sensor as cited in  claim 1 , wherein said side narrow sections of the thin film bridge are made SiO2/Si3N4 and have a beam shape with a length ranging from 200 to 800 microns and a thickness ranging from 1 to 2 microns. 
     
     
       6. The method of manufacturing the single silicon wafer micromechanical thermal conduction sensor as cited in  claim 1 , wherein said wide central section of the thin film bridge is made of Si3N4/polysilicon/Si3N4 and has a square shape with a side length ranging from 200 to 1000 microns and a thickness ranging from 1 to 3 microns. 
     
     
       7. The method of manufacturing the single silicon wafer micromachined thermal conduction sensor as recited in  claim 1 , wherein said heated resistor is made of Nickel or Platinum and has a resistance ranging from 50 to 5000 ohm. 
     
     
       8. The method of manufacturing the single silicon wafer micromachined thermal conduction sensor as recited in  claim 1 , wherein said temperature sensor is made of Nickel or Platinum and has a resistance ranging from 50 to 5000 ohm.